Mechanically reducing the size of ductile metal powders using conventional stirred ball mills yields a product which is typically "flaky" in morphology. This is due to a great extent to the design of the milling media agitators, which are generally constructed in such a way that the media is vigorously lifted up while being stirred. This leads to a lower than desired media packing density, extensive media-media impact, and a "flaky" final product.
Previous efforts in fine grinding of ductile metal powders fall into two categories: (1) metal flake production for coatings and paints, and (2) mechanical alloying.
The former is a very straightforward process. Metal powders are processed in either a rotary (tumbling) ball mill or a stirred ball mill as slurry with either water or an organic solvent. Usually the slurry has additions of an organic compound which adsorbs on newly exposed powder surfaces and reduces or prevents welding or agglomeration of the particles. Impact is maximized by the selection of milling parameters, and the resulting product has a very thin "flaky" morphology (i.e. diameter: thicknesss ratios &gt;20-50.) This is desirable for this application, since one intent of the product is to produce a coating consisting of overlapping thin flakes of metal bound together by an organic resin or polymer.
In mechanical alloying, metal powders, with or without additions of metallic oxides, are processed for very long times to achieve mixing of the components on an atomic scale. True alloys (as measured by x-ray diffraction) may be produced from elemental components by using this process. Component powders are processed dry in either a rotary ball mill or a stirred ball mill. Again conditions are chosen so that maximal impact occurs. The particles typically flatten to a flaky morphology, are mechanically welded or forged back together into agglomerates, and the agglomerates are broken down in size. This three step action of mechanical work, agglomeration, and fracture eventually yields roughly equiaxed particles. However, processing times are long to achieve an equilibrium state and there is always the risk of overheating the powder and welding the media and powder into a solid mass. The long processing times required for mechanical alloying are not necessary nor are they desirable for size reduction of pre-alloyed metal powders. The production rate can be shortened extensively by not processing to an equilibrium state, as long as the particles develop and maintain a roughly equiaxed morphology.